An atomic force microscope (hereinafter, also referred to as AFM) detects a force of interaction working between the surface of a sample and a probe held near the surface of the sample. Moreover, the AFM performs feedback control for the distance between the probe and the sample to keep the detected force of interaction constant. Furthermore, the AFM horizontally scans the sample while maintaining the feedback control. This allows the probe to go up and down along the surface asperities of the sample with a certain distance between the probe and the sample. The AFM can obtain a topographic image of the surface of the sample by recording the vertical movement of the probe with respect to the scanned horizontal position.
Here, a cantilever having the probe at its one end is used for a force sensor for detecting a force of interaction between the probe and the sample. An optical lever method, for example, is widely used for detecting the displacement magnitude of this cantilever. In the optical lever method, the displacement magnitude of the cantilever is detected by irradiating the back side of the cantilever with a light beam such as a laser beam and detecting light reflected off the back side with a position detection sensor such as a four-split photodiode.
The AFM is especially useful as a method for directly observing the shape and physical properties of the surface of a sample in a liquid. Hereinafter, an AFM for observing a sample in a liquid is referred to as liquid AFM. The liquid AFM measures the surface of a sample in the state where the cantilever and the sample are immersed in an observation liquid.
Here, a problem is in that evaporation of the observation liquid constantly changes the concentration of dissolved matter. Moreover, an increase in the amount of evaporation changes the solution shape of the observation liquid. Thus, a laser beam path may change. This makes it difficult to detect the accurate displacement magnitude of the cantilever.
In view of the problems, an approach to seal a space in an AFM cell with an elastic body such as a rubber O ring has been suggested (see Patent Literature 1 (PTL 1). Evaporation can be prevented by sealing an observation liquid with the elastic body.
Moreover, an approach to prevent evaporation of an observation liquid by filling the surroundings of the observation liquid with a sealing liquid which does not mix with the observation liquid has been suggested (see Patent Literature 2 (PTL 2)).